Physical-chemical techniques and site specific mutagenesis will be employed to study the structural basis for the establishment of the stereospecific sites in the regulatory protein biosynthetic L-threonine deaminase. The basis for the differential expression of the ilvGEDA gene will be studied employing in vitro derived recombinant plasmids and nucleotide sequence analysis. Also, the basis for a high frequency switching mechanism which converts the regulation of the ilvB gene from attenuation to catabolite repression will be investigated.